1. Field of the Invention
The present invention relates to a side door structure and a method of designing the side door structure applicable to a vehicle body of which side openings are covered by front doors swingably supported by door hinges located at the front of the front doors and by rear doors swingably supported by door hinges located at the rear of the rear doors.
2. Description of the Related Art
There exists a conventionally known side door structure of a vehicle of which front doors and rear doors covering side openings in a vehicle body are opened frontward and rearward, respectively, as described in Japanese Unexamined Patent Publication No. 2001-138864, for example. In this side door structure (hereinafter referred to as the two-way opening side door structure), the front doors are swingably supported by door hinges located at the front of the front doors and the rear doors are swingably supported by door hinges located at the rear of the rear doors so that they can be opened frontward and rearward, respectively.
The prior art to which the invention is directed also discloses a door checker interconnecting a vehicle body and a door for controlling door swinging torque and for holding the door at a specific opening angle. This kind of door checker disclosed in Japanese Unexamined Patent Publication No. 2001-138864, for instance, includes a checker plate affixed to the vehicle body or the door, and is located between upper and lower door hinges which serve as a supporting point of the swinging door.
The two-way opening side door structure of the vehicle disclosed in the above-cited Japanese Unexamined Patent Publication No. 2001-138864 is built as follows. When opening a rear door 4a, an occupant first undoes a door lock of a front door 2a and opens the front door 2a to a specific angle as shown by imaginary lines in FIG. 20 to release the front and rear doors 2a, 4a from their interlocked state. Then, the occupant swings the rear door 4a from a closed position to an open position.
Generally, upper portions of side doors (front and rear) of the vehicle are inwardly inclined as viewed from the front of the vehicle so that upper ends of the side doors are located more or less inward toward a center line of the vehicle body largely for design-related reasons. When the front and rear doors 2a, 4a of which upper portions are inwardly inclined are swingably supported by vertically mounted hinge pins 17a and 12a, respectively, as shown in FIG. 21, a rearmost part of the upper end of the front door 2a swings slightly rearward whereas a forwardmost part of the upper end of the rear door 4a swings slightly frontward in an initial stage of opening the respective doors 2a, 4a. This is because the upper ends of the front and rear doors 2a, 4a are positioned inward toward the center line of the vehicle body with respect to longitudinal axes of the hinge pins 17a, 12a. 
In addition, the front and rear doors 2a, 4a are installed in such a manner that a rear end of the front door 2a and a front end of the rear door 4a overlap each other by a specific distance OL. Therefore, a swing trajectory of the rear end of the front door 2a and a swing trajectory of the front end of the rear door 4a produced as the respective doors 2a, 4a are opened and closed inevitably overlap over a large area “A” as illustrated (hatched in FIG. 21).
In this structure, the front end of the rear door 4a is apt to interfere with the rear end of the front door 2a when the rear door 4a is opened and, therefore, the occupant has to open the rear door 4a after widely opening the front door 2a to prevent this interference between the front and rear doors 2a, 4a. The conventional two-way opening side door structure has this problem concerning operational ease.
One approach to the prevention of the interference between the front and rear doors 2a, 4a which could be caused by the large overlapping area “A” of their swing trajectories would be to tilt the hinge pins 17a, 12a of the front and rear doors 2a, 4a by an amount corresponding to the inclination of their upper portions. If the hinge pins 17a, 12a are so inclined, however, there arises a problem that a large force would be needed for opening the doors 2a, 4a from their closed positions. FIG. 21 shows a state of the front and rear doors 2a, 4a as viewed along an axial direction of the hinge pins 17a, 12a which are inclined such that upper ends of the hinge pins 17a, 12a are inwardly offset.
A comparison between FIGS. 20 and 21 indicates that the overlapping area “A” of the swing trajectories of the front and rear doors 2a, 4a produced as the doors 2a, 4a are swung about the respective hinge pins 17a, 12a can be reduced by tilting the hinge pins 17a, 12a by the amount corresponding to the inclination of the upper portions of the respective doors 2a, 4a. If the hinge pins 17a, 12a are inclined such that their upper ends are inwardly offset as mentioned above, however, large moments of force act on the front and rear doors 2a, 4a in their closing direction, wherein the moment of force (resisting moment) acting on each door 2a, 4a is given as the product of the weight M of each door 2a, 4a acting vertically downward through its center of gravity O and a distance L1, L2 from the center of gravity O to the hinge pin 17a, 12a. One problem of this approach (tilting of the hinge pins 17a, 12a) is that a large force is needed in an initial stage of opening the individual doors 2a, 4a, because the occupant has to open the doors 2a, 4a against the large moments of force.
Another problem of the aforementioned conventional approach is that, because the resisting moment works on the front and rear doors 2a, 4a until they reach an opening angle θ of approximately 90 degrees, specific amounts of force are needed up to a final stage of opening the two doors 2a, 4a and it is difficult to hold the individual doors 2a, 4a at their fully open positions.
Although it would be preferable to hold the rear door 4a at an opening angle convenient for a rear seat occupant to get into and out of the vehicle by means of a door checker as disclosed in the aforementioned Japanese Unexamined Patent Publication No. 2001-138864, for example, it is necessary to use a door checker capable of exerting a considerable restraining force for retaining the rear door 4a at such an opening angle in a stable fashion. It is however difficult to ensure a space for installing this kind of door checker, and such a door checker imposes significant limitations on the degree of freedom in component layout.